CHICAGO (Reuters) — US researchers think they now understand why some cancers grow back after radiation treatment.

They said a specialized type of cell known as a cancer stem cell has a protective mechanism that keeps radiation from damaging the DNA and proteins inside the cell.

And by jamming this protective mechanism, they think they may be able to make cancer treatments work better, the team reported in the journal Nature. "Our ultimate goal is to come up with a therapy that knocks out the cancer stem cells," said Dr. Robert Cho of Stanford University School of Medicine, who worked on the study published in the journal Nature.

Several research teams are focusing on cancer stem cells, a kind of master cell that appears to have the ability to replenish tumour cells. Finding ways to destroy these cells could make cancer far easier to treat. "It's like battling weeds," Cho said of cancer stem cells' ability to regenerate. "You can go through a big field with a weed whacker, but the weeds are going to come back unless you get the roots."

Cancer stem cells appear to resist conventional cancer treatments like radiation, and Cho and colleagues wanted to know why. In a series of experiments using cells from mice and humans, the Stanford team found that breast cancer stem cells make much higher levels of protective proteins known as antioxidants than other cancer cells.

These antioxidants can protect the DNA and proteins from the effects of harmful substances known as reactive oxygen species, otherwise called oxidative stress. The researchers believe this protective mechanism helped explain why breast cancer stem cells were less likely than other cancer cells to succumb to radiation. When the team used a drug to block an antioxidant protein called glutathione, the cancer stem cells became far more vulnerable to radiation.

"Essentially, it inactivated the cancer stem cells' protective mechanism," team-member Michael Clarke of Stanford University said in a telephone interview. The team now hopes to find drugs that can specifically disarm that protective mechanism in cancer stem cells while sparing normal cells.